optimize: merge nat rules with same selectors into map

optimize: merge nat rules with same selectors into map

Verdict and nat are mutually exclusive, no need to support for this
combination.

 # cat ruleset.nft
 table ip x {
        chain y {
		type nat hook postrouting priority srcnat; policy drop;
                ip saddr 1.1.1.1 tcp dport 8000 snat to 4.4.4.4:80
                ip saddr 2.2.2.2 tcp dport 8001 snat to 5.5.5.5:90
        }
 }

 # nft -o -c -f ruleset.nft
 Merging:
 ruleset.nft:4:3-52:                ip saddr 1.1.1.1 tcp dport 8000 snat to 4.4.4.4:80
 ruleset.nft:5:3-52:                ip saddr 2.2.2.2 tcp dport 8001 snat to 5.5.5.5:90
 into:
        snat to ip saddr . tcp dport map { 1.1.1.1 . 8000 : 4.4.4.4 . 80, 2.2.2.2 . 8001 : 5.5.5.5 . 90 }

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>